The present invention relates to a method for operating a helium leak indicator having the characteristics of patent claim 1. Moreover, the present invention relates to a helium leak indicator suited for the implementation of this method.
During helium leak searching, the detection limit may be improved by reducing the pumping capacity of the vacuum pumps. For the purpose of reducing the pumping capacity it is known to use constrictions which may be switched on, or to disconnect the backing pump. In both instances the pumping capacity is reduced by increments and with a likewise increase in sensitivity. The price to pay for such an increase in sensitivity are longer measurement times, since these increase when reducing the pumping capacity of the vacuum pumps.
It is the task of the present invention to simplify, within the scope of a method for operating a helium leak indicator of the kind affected here, the way in which the pumping capacity is influenced and thus the way in which the detection limit of the leak measurements is attained, in order to perform high sensitivity measurements within an optimally short time. Moreover, an infinitely variable modification of the detection limit shall become possible.
According to the present invention these targets are attained through the characteristic features of the patent claims.
The present invention allows, through a modification of the modulation properties of the gas flow in which the presence of the test gas is to be detected, to influence the effective pumping capacity of the vacuum pumps, and thus the sensitivity of the leak search. Thus it becomes possible to the user to adapt the detection limit to the particular application in each case by setting up certain modulation properties. This adaptation allows the use of optimally short measurement times in each case. Since according to the present invention the pumping capacity is influenced with the aid of an inlet valve which can be modulated, any special measures for modifying the pumping capacity at the vacuum pumps themselves are no longer required.
An expedient operating method which has become possible through the present invention is, that the detection limit may be modified automatically down to a lower limit while the leak measurement is in progress. As the manipulated variable the signal supplied by the mass spectrometer may be employed, the magnitude of which depends, in the case of a not yet registered leak, on the helium background in the gas flow which is to be analysed, and which reduces in magnitude with increasing measurement time. As long as the signal magnitude does not drop below a given level, the modulation properties are not modified, and a relatively high pumping capacity is maintained. As soon as the magnitude of the signal drops below a given threshold, the pumping capacity is reduced by modifying the modulation properties and thus sensitivity is increased. Each leak determined and measured in this manner may therefore be determined within an optimally short measurement time.